Many, for example, buried pipelines, such as sewer lines, and among these in particular concrete sewer pipes, deteriorate gradually, and for various reasons also other damage may appear in them, such as cracks, ruptures, and the like. At some stage the pipelines in use must be repaired. This can, of course, in the case of sewer lines be done by digging them up and by replacing them with new pipes. However, the excavation and filling work is expensive and time-consuming, and additionally the excavations cause disturbance to traffic, for example in cities. For this reason, damaged pipelines are now being repaired by means of so-called relining pipes which are, for example, plastic pipes which are inserted from the end of the pipeline section to be repaired into the original pipe, whereby the original pipe is lined inside.
There are several types of such relining pipes. One possibility is to use flexible, for example corrugated, plastic piping, the parts of which are interconnected by welding above ground and the pipe is inserted in a continuous form, for example, through a manhole, into the pipeline section to be repaired. Such piping is inexpensive, but it can be used only in relatively small diameters, since a large-diameter pipe cannot flex in the small space of a manhole. In addition, in such a pipe structure it is difficult to arrange a branching system which could be implemented without opening the intersection area.
Another possibility is to use pipe elements the length of which is at maximum the diameter of a manhole, in which case the pipe elements can be inserted one at a time to extend a pipe element or a row of pipe elements already in the sewer line section, and this relining pipe made up of pipe elements is inserted always one element length at a time into the sewer line section being repaired. One possibility for forming such pipe elements is to cast or otherwise form tubular pieces having female sleeves at both ends. In addition, in this embodiment male pipe lengths are used the length of which corresponds to the length of two successively placed sleeve parts. In this case, the outermost ends of the female parts will settle against each other and the pipe length inside them will keep the ends of the elements in place. This relining pipe is thicker at the female parts than elsewhere. However, this structure has the disadvantage that counter-surfaces which bear the compression force generated between the pipe elements when they are being inserted into the sewer consist only of the said female-part ends, i.e. one point. In the contact point area of the pipe length and the base of the female part there are no surfaces resisting the push, and so the pipe length may slide through the female part to the inside of the element, thereby damaging the structure. Furthermore, this construction has no tension-resistent shaping by means of which the elements becoming detached from each other would be prevented if they for some reason have to be pulled backwards even over a small distance. The result may be a pipe lining having one or more detached joints at unknown locations, in which case the relining is to be deemed a failure.
Another relining-pipe structure is made up of elements which were originally formed from extruded piping by cutting it into suitable lengths and by machining at one end of each of the elements thus obtained a female sleeve part and at the other end a male locking part. In this known structure there are, in the outer edge of the male locking part and in the bottom of the female part, snap-locking counterparts which will prevent the pipe elements from becoming detached from each other under tension. In addition, in this structure it is relatively easy to produce two sets of counter-surfaces resisting pressure, whereupon the couplings between the pipe elements will withstand pushing relatively well. However, this structure is expensive, since the pipe has to be extruded so as to have very thick walls in order to enable a sufficiently sturdy female sleeve part and a male locking part to be machined in it. In this case the material costs are very high. In addition, the manufacture of the female part and the male part entirely by machining increases the price and occasionally causes dimensional errors. These dimensional errors in the female part or the male part complicate installation, or sometimes even make it impossible. On the other hand, if the extruded pipe were made with thinner walls, the wall of the female part and the wall of the male part would be so thin that the joint would break when the pipes are pressed together. In this alternative the relining pipe in its entirety has a smooth exterior surface.